Normal everyday washing only requires fresh water and you may wish to use a form of marine wash. If your Polycraft is really grubby you can use engine degreaser, truck wash, or CT18 which is available at most Supercheap Auto stores and wash down with a high pressure hose.
NOTE: Do not try and polish your boat with car polish or any type of buffing compound, this will have the reverse effect. Once the boat has been washed thoroughly you can buff the boat with a lambswool buffing pad (no liquids).